A piezoelectric element with a piezoelectric composition used therein has an effect of generating a deformation when an external electric field is applied, and also has an effect of generating electric charges on the surface when it receives an external stress. Recently, such a piezoelectric element is widely used in various fields.
For example, a piezoelectric element which uses a piezoelectric composition such as lead zirconate titanate (Ph(Zr, Ti)O3:PZT) and the like will deform in proportion to the applied electric voltage with the displacement value being in a level of about 1×10−10 m/V. Thus, such a piezoelectric element is excellent in fine position adjustment and can be further used for fine adjustment m an optical system.
Besides, as a piezoelectric composition will generate electric charges in proportion to the applied stress or the deformation caused by the stress, it also can be employed as a sensor for detecting minute forces or the extent of deformations.
Furthermore, a piezoelectric composition has an excellent responsiveness. Thus, when an alternating current field is applied, resonance could occur due to the excitation of the piezoelectric composition itself or an elastic body coupled with the piezoelectric composition. In this respect, a piezoelectric composition can also be used as a piezoelectric transformer, a ultrasonic motor and etc.
Most of these piezoelectric compositions have a perovskite structure. The piezoelectric composition is formed by applying a polarization treatment to a ferroelectric composition. The polarization treatment is such a operation that a high direct current electric field is applied to a isotropic ferroelectric ceramics obtained after firing so as to render the direction of the ferroelectric domain to be consistent in a certain direction, and thus the ferroelectric composition is provided with a polarity. Since the perovskite structure with the spontaneous polarization capable of being three dimensionally oriented is quite helpful, most piezoelectric compositions in practical use have the perovskite structure.
Currently, most of the piezoelectric compositions in practical use are (PZT-based) solid solutions composed of PbZrO3(PZ)-PbTiO3(PT). The piezoelectric compositions meeting various needs can be widely developed by adding various accessory ingredients or additives into the PZT-based piezoelectric composition.
There are different piezoelectric compositions such as a piezoelectric composition with a low mechanical quality factor (Qm) and a high piezoelectric constant (d), and a piezoelectric composition with a low piezoelectric constant (d) and a high mechanical quality factor (Qm). The previous one is used in an actuator or the like for position adjustment which seeks a large displacement via a direct current based usage. The latter one is applicable to alternating current related uses. For example, the latter one is used in an ultrasonic generating element such as an ultrasonic motor.
In addition, there are substances other than PZT-based ones that can be used as piezoelectric compositions, most of which are solid solutions using a lead based perovskite component such as lead magnesio-niobate (Pb(Mg,Nb)O3:PMN) or the like as the main component.
However, these lead based piezoelectric compositions contain 60 to 70 mass % of lead oxides with a low melting point which is likely to volatilize during, the firing process. If the influence on the environment is considered, it is expected that less lead oxides will be used.
Therefore, the lead-free piezoelectric composition will become an extremely important issue if the piezoelectric ceramics and the piezoelectric single-crystals are to be applied to more fields and to be used in more amounts.
As a lead-free piezoelectric composition, for example, barium titanate (BaTiO3), the bismuth layered ferroelectric or the like are well known. However, since the barium titanate has a curie point as low as 120° C. and its piezoelectric property will disappear at a temperature higher than 120° C., it will not be practical if it is bonded by welding or used in vehicles. On the other hand, although the bismuth layered ferroelectric usually has a curie point of 400° C. or higher and is excellent in thermal stability, its crystal anisotropy is high. Thus, it is necessary to use a method, such as a hot forging method, in which spontaneous polarization is oriented by applying shear stress while heating and sintering the bismuth layered ferroelectric. Hence, problems arise in the aspect of productivity.
Recently, the bismuth sodium titanate based composition is being studied as a new piezoelectric composition. For instance. Patent Document 1 has disclosed a piezoelectric ceramic composition containing bismuth sodium titanate.
The piezoelectric ceramic composition in Patent Document 1 contains a matrix material and the matrix material contains at least two matrix components with a perovskite structure. Alternatively, the piezoelectric ceramic composition is only composed of the matrix component. Further, a piezoelectric ceramic composition is disclosed in which a first matrix component is selected from the group consisting of (Bi0.5A0.5)EO3 and BaEO3 and the other matrix component is Bi(Me0.5E0.5)O3, wherein A represents an alkali metal and is especially selected from the group consisting of sodium, potassium and a mixture of alkali metals, E is independently selected from titanium, zirconium and their mixture, and Me is selected from the group consisting of bivalent metals.